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Period of susceptibility of almonds to aflatoxin contamination during development in the orchard

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Abstract

Almonds can be contaminated with aflatoxins, produced mainly by Aspergillus flavus and A. parasiticus. Infection can be facilitated by insect injuries during hull split, which begins four to six weeks before harvest. Within this period, it is unknown which kernel stages are most susceptible to aflatoxin contamination. Developing almonds of the Nonpareil cultivar were inoculated weekly with a spore suspension of A. flavus or A. parasiticus for five weeks after hull split in 2013. The almonds were infested with eggs of the lepidopteron navel orangeworm (NOW) (Amyelois transitella) before each spore inoculation. Aflatoxin levels were quantified at harvest using HPLC. Aflatoxin contamination was consistently higher in NOW-damaged kernels, although aflatoxins were also detected in undamaged kernels at each inoculation date. Insect injury is not required for kernel infection but it is a key risk factor for high aflatoxin contamination. Laboratory inoculations were also performed on Nonpareil almond kernels collected during the summers of 2013 and 2015. Aflatoxin levels were significantly lower on dried almonds but the ability to produce aflatoxins was restored when almonds were incubated with high humidity or when the Aspergillus species were inoculated on almond meal agar amended with ground kernels. Therefore, aflatoxins can accumulate in kernels with low aw, should sufficient moisture favors aflatoxin production. In our field experiment, the orchard micro-climate had sufficient humidity to enable aflatoxin production in both damaged and undamaged dried kernels.

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Acknowledgements

The authors are very grateful to Patricia Noble for providing NOW eggs and to Michael Luna for his help in the field experiments. UC Davis and USDA/ARS are equal opportunity employers.

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Author notes

  1. Adeline Picot

    Present address: Université de Brest, EA 3882, Laboratoire Universitaire de Biodiversité et Ecologie Microbienne, IBSAM, ESIAB, Technopôle Brest-Iroise, 29280, Plouzané, France

Authors and Affiliations

  1. Department of Plant Pathology, University of California Davis, Kearney Agricultural Research and Extension Center, Parlier, 93648, CA, USA

    Adeline Picot, Alejandro Ortega-Beltran, Ryan D. Puckett & Themis J. Michailides

  2. USDA-ARS, San Joaquin Valley Agricultural Sciences Center, Parlier, 93648, CA, USA

    Joel P. Siegel

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  1. Adeline Picot
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  2. Alejandro Ortega-Beltran
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  5. Themis J. Michailides
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Correspondence to Themis J. Michailides.

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Picot, A., Ortega-Beltran, A., Puckett, R.D. et al. Period of susceptibility of almonds to aflatoxin contamination during development in the orchard. Eur J Plant Pathol 148, 521–531 (2017). https://doi.org/10.1007/s10658-016-1108-2

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